In a large scale MHD channel, it will become important to get information on the fluctuations of the electron density and electrical conductivity, and on the correlation between them. The far-infrared (FIR) laser has offered a powerful diagnostic tool for this purpose, and the method was establish by this research group that enabled the simultaneous measurements of electron density and conductivity. The purpose of this research project was to improve FIR-laser method to be applicable to the large-scale experimental facility by using Faraday rotation principle. While the experimental research was being done, however, the new interferometer was devised that could measure the large fluctuation of the electron density and conductivity. This interferometer adds another interferometric path to the conventional Michelson interferometer, and is capable of measuring the large fluctuation with the fringe shifts over 2pi. Therefore, the purpose of this project was changed slightly to clarify the characteristics of this new interferometer, and the following results were obtained.1. To establish the stable FIR interferometer, the new HCN laser was designed and made. This laser was proved to operate successfully and shown to be used in the interferometer system.2. The interferometer based on the entirely new concept was designed and made. This has one more interferometer path in the conventional Michelson interferometer. Also, the Schottky-barrier-diode was used to detect the fluctuation. The software that took into account the multiple interferometer path was made. It was shown that the present new system could successfully measure the fluctuations of the electron density and conductivity.3・灯油燃焼チャネルで得られた燃焼ガスプラズマを対象とし、この二重干渉計の実験的研究を行った。MHDプラズマでは初めてレーザービーム検出器にショットキバリアダイオードを使用し、これまでに例のない高時間分解能測定を実現した。その結果、本方法で電子密度と導電率の変動が高精度で測定可能であることを実証できた。